Project Summary/Abstract The formation of the intricate neuronal network in the brain requires precise neurogenesis and neuronal migration followed by neurite and spine formation during development. If neurite and spine formation is disrupted, it results in a wide range of diseases such as mental illnesses. Recently, patients with a microduplication of the 17p13.3 chromosome region have been reported and the number of patients exhibiting this syndrome has been increasing. Interestingly, the patients with this 17p13.3 microduplication syndrome show severe neurological and morphological defects, including, epilepsy, mental retardation and autism spectrum disorders (ASD). More interestingly, the studies analyzing this new human genetic syndrome clarified that the critical region spanning about 70kb is strongly associated with ASD, and this region contains a single gene, Ywhae, encoding the protein 14-3-3?. These data strongly suggest that Ywhae might be responsible for the ASD phenotype in these patients. The functions of 14-3-3? in neuronal morphogenesis including neurite formation, have not been previously analyzed and still remain unknown. Therefore, the objectives of this research proposal are to analyze the novel in vivo functions and molecular targets of 14-3-3? in neurite formation, in particular neurite initiation and extension, and clarifying the molecular mechanisms in this cellular event. We hypothesize that 14-3-3? plays an important role in neurite initiation as well as neurite extension by controlling microtubule (MT) sliding and stability through binding to Doublecortin (Dcx) and Microtubule affinity regulating kinase 3 (Mark3). To test this hypothesis, we propose to use a wide variety of experimental strategies including in utero electroporation, time-lapse live imaging in vitro and in vivo, tamoxifen-inducible Cre-loxP system, and 14-3-3? conditional knockout mice. We have three specific aims. In the Specific Aim 1, we will test the hypothesis that 14-3-3? regulates neurite formation and synaptogenesis. In Specific Aim 2, the hypothesis that 14-3-3? regulates neurite initiation in the cortex by regulating MT sliding through binding to Dcx will be tested. In Specific Aim 3, we will test the hypothesis that 14-3-3? regulates neurite extension by binding to Mark3 and regulating its activity. The successful completion of this project will not only provide understanding as to the etiology of a new human genetic syndrome strongly associating with ASD caused by the overexpression of 14-3-3? but also significantly enhance our understanding of the precise in vivo functions of 14-3-3? in neurite initiation and extension.